Many semiconductor devices are formed by processes performed on a substrate. For example, in the fabrication of light-emitting diodes (LEDs) and other high-performance devices such as laser diodes, optical detectors, and field effect transistors, a chemical vapor deposition (CVD) process is typically used to grow a thin film stack structure using materials such as gallium nitride over a sapphire or silicon substrate. The substrate typically is slab of a crystalline material, commonly referred to as a “wafer,” typically in the form of a disc.
One common process is epitaxial growth. For example, devices formed from compound semiconductors such as III-V semiconductors typically are formed by growing successive layers of the compound semiconductor using metal organic chemical vapor deposition or “MOCVD.” In this process, the wafers are exposed to a combination of gases, typically including a metal organic compound as a source of a group III metal, and also including a source of a group V element which flow over the surface of the wafer while the wafer is maintained at an elevated temperature. Typically, the metal organic compound and group V source are combined with a carrier gas which does not participate appreciably in the reaction as, for example, nitrogen. One example of a III-V semiconductor is gallium nitride, which can be formed by reaction of an organo gallium compound and ammonia on a substrate having a suitable crystal lattice spacing, as for example, a sapphire wafer. Typically, the wafer is maintained at a temperature on the order of 1000-1100° C. during deposition of gallium nitride and related compounds.
Composite devices can be fabricated by depositing numerous layers in succession on the surface of the wafer under slightly different reaction conditions, as for example, additions of other group III or group V elements to vary the crystal structure and band gap of the semiconductor. For example, in a gallium nitride based semiconductor, indium, aluminum or both can be used in varying proportion to vary the band gap of the semiconductor. Also, p-type or n-type dopants can be added to control the conductivity of each layer. After all of the semiconductor layers have been formed and, typically, after appropriate electric contacts have been applied, the wafer is cut into individual devices. Devices such as light-emitting diodes (“LEDs”), lasers, and other optoelectronic devices can be fabricated in this way.
The epitaxial growth process can be carried out in a CVD tool that includes a process or reaction chamber, which provides a sealed environment that allows infused gases to be deposited upon the substrate to grow the thin film layers.
One type of CVD tool which has been widely accepted in the industry uses a device commonly referred to as a “wafer carrier.” Wafer carriers typically comprise a large disc with numerous wafer retaining regions or “pockets,” each pocket adapted to hold one wafer. The wafer retaining pockets are typically comprise a recess or cavity formed on the top surface dimensioned to receive a wafer, and are characterized as having an upwardly-facing floor and a radially inward-facing perimeter wall.
Typically, the wafers are supported by protrusions that extend from the floor and the perimeter wall the wafer retaining pocket to suspend the wafer above the surface of the floor of the wafer retaining pocket. The wafer can bow downward (concave) in different process layers. Suspension above the pocket floor at a predetermined distance prevents wafer from unevenly sitting bottomed out in the pocket due to this shape, which can cause wafer toss and large temperature non-uniformities. Maintaining a certain distance between the wafer and the pocket floor also reduces the wafer temperature non-uniformity that results from an uneven wafer-pocket gap due to the bowing of the wafer. The suspension of the wafer also promotes uniformity of the temperature of the wafer by eliminating random contact points that would otherwise result between the wafer and the pocket floor. Moreover, certain wafer carriers implement pocket floors that are appropriately bowed or shaped to help with uniformity at certain process layers. If the wafer rests on the floor directly, layers that the floors were not designed for can incur large non-uniformities due to bottoming out.
The wafer carrier is supported on a spindle within the reaction chamber so that the top surface of the wafer carrier with exposed surfaces of the wafers is oriented to face upwards, toward a gas distribution element of the CVD tool. While the spindle is rotated, the gas is directed downwardly onto the top surface of the wafer carrier and flows across the top surface toward the periphery of the wafer carrier. The wafer carrier is maintained at the desired elevated temperature by heating elements, typically electrical resistive heating elements disposed below the bottom surface of the wafer carrier. These heating elements are maintained at a temperature above the desired temperature of the wafer surfaces, whereas the gas distribution element typically is maintained at a temperature well below the desired reaction temperature so as to prevent premature reaction of the gases. Therefore, heat is transferred from the heating elements to the bottom surface of the wafer carrier and flows upwardly through the wafer carrier to the individual wafers.
The importance of maintaining uniform conditions at all points on the top surfaces of the various wafers during the CVD process has long been recognized. Minor variations in composition of the reactive gases and in the temperature of the wafer surfaces cause undesired variations in the properties of the resulting semiconductor device. For example, if a gallium and indium nitride layer is deposited, variations in wafer surface temperature will cause variations in the composition and band gap of the deposited layer. Because indium has a relatively high vapor pressure, the deposited layer will have a lower proportion of indium and a greater band gap in those regions of the wafer where the surface temperature is higher. If the deposited layer is an active, light-emitting layer of an LED structure, the emission wavelength of the LEDs formed from the wafer will also vary. Thus, considerable effort has been devoted in the art heretofore towards maintaining uniform conditions.
While considerable effort has been devoted to design an optimization of such systems, further improvement would be desirable. In particular, it is desirable to provide better uniformity of temperature across the surface of each wafer.